The proposed research is planned to utilize the unique opportunities for genetic studies of tumorigenesis provided by the CHEF cell line of Chinese hamster embryo fibroblasts which we have developed. 1) transformed and tumor forming cell lines have been recovered after treatment of CHEF cells with the mutagens EMS and MNNG. These cell lines are now being used for genetic mapping studies to identify genes associated with growth factor requirement, the anchorage requirement for growth, and tumor forming ability. Identification of chromosomes carrying genes that influence tumor forming ability is being carried out with hybrid subclones that have undergone chromosome reduction. 2) the mechanism of suppression of tumorigenicity in cell hybrids is under continuing investigation. 3) new markers are being isolated and will be mapped onto the Chinese hamster chromosomes to be used in the process of mapping specific genes involved in transformation and tumorigenicity.